1. Field of the Invention
The present invention relates to a method of injection-molding for epoxy resin moldings and to an injection-moldable epoxy resin composition. More particularly, the present invention relates to a method of injection-molding for epoxy rein moldings wherein a mini-sprue metal mold is used and to an injection-moldable epoxy resin composition suitable for such method possessing latent-type curing characteristics.
2. Description of the Related Art
It is well known that epoxy resins are excellent in various physical properties such as heat-resisting property, abrasion-resisting property, chemicals-resisting property and the like and are employed for various uses.
From the past, a transfer molding method is known as a means for molding epoxy resin materials which is appropriate in the aspect of productivity. In transfer molding, it is ordinary that an epoxy resin composition is cast in a pot of a metal mold prior to molding and the composition is then pressed by a plunger and cured under fluidization within the metal mold.
In this molding method, however, a material for molding has initially to be shaped into tablets and a preheating step is needed so that shortening of the molding cycle cannot be made over a given rate, thus making productivity and economization of cost naturally limitative. In transfer molding, moreover, there remains such a problem in the course of molding that a cull part remaining in the pot in addition to a runner part becomes a waste portion after curing.
The cured cull part is normally discarded as a waste without being recycled, but the waste is not decomposed and thus causes an environmental pollution.
In an injection-molding method, on the other hand, a thermoplastic resin powder for molding is only charged into a hopper whereby the subsequent preheating and weighing operations of the powder are automatically operated in a cylinder and the fluidized rein is injected into a metal mold where curing of the resin is effected. In this case, therefore, steps for shaping the molding material into tablets and preheating them are not required and furthermore a continuous production of the moldings becomes possible, thus making shortening of the molding cycle possible as compared with the transfer molding operation. The injection-molding method can thus be said to be superior in the aspect of production efficiency.
However, epoxy resin molding materials employed heretofore in transfer molding are inferior in thermal stability in the cylinder so that melt viscosity of the materials increases extremely to make injection-molding unable.
As a metal mold for injection-molding of epoxy resins, there are known a conventionally used metal mold for injection-molding and a mini-sprue metal mold as shown in FIG. 2 which comprises a manifold part, a mini-sprue part and an ejector plate part. In a conventional metal mold for injection-molding, a part corresponding to the manifold part of the mini-sprue metal mold functioned as a sprue and a runner and was maintained together at a temperature where curing of the resin was promoted so that the whole manifold part and the sprue part were taken out together in cured state for every molding cycle.
Such a problem on the injection-molding may also take place in case of the transfer molding. As the metal mold is maintained as a whole at a temperature capable of promoting curing, the cull part and the runner part are taken out in cured state together with the moldings.
As the epoxy resin is a thermocuring resin, it cannot be recycled for use once it has been cured so that the above cured portions incidentally formed with the product produced on molding have to be discarded as such. The epoxy resin is longer in molding cycle as compared with the thermoplastic resin so that a number of problems to be improved remain also in the aspect of productivity.